The present invention relates to a method for guiding a missile fired at a target, where the angular velocity of the target is determined on the basis of the operator tracking the target in a first time period during which at least a first angle position and a second angle position of the target are recorded and the time interval between these, and where, based on the determined angular velocity, the angle position which the target is assumed to have when the missile reaches the target is predicted and the missile is guided continuously in a desired, predicted trajectory towards the assumed angle position as a function of time and missile speed. The invention also relates to a guidance system for guiding a missile, comprising means for determining the angular velocity of the target in a first time period before when the operator is tracking the target, based on the recording of a first angle position and a second angle position and the time interval between these, means for predicting the position of the target when the missile is expected to reach the target, based on the determined angular velocity, and means for predicting the desired trajectory. In this text, the term missile is intended to cover all forms of internally and/or externally guided objects which are fired at a target. An example of a suitable type of missile is an anti-tank missile.
The traditional way of getting the payload of a portable anti-tank weapon in position is to fire it in a ballistic trajectory to the target. Problems which arise in doing this include:
Operator variations in sighting and firing.
Difficulties in determining the target distance and speed.
Unknown or difficult atmospheric conditions.
Ballistic spread.
Changes in the target""s movements.
Certain of the above difficulties are more important when it is necessary to fire from enclosed areas. The low muzzle velocity considerably reduces the maximum range of the weapon. A modern weapon with demands for increased maximum range must in some way overcome the difficulties indicated above. Some form of guidance of the fired missile during its travel to the target is needed.
It has previously been proposed, for medium distances, to use a guidance method known by the term Predicted Line of Sight (PLOS). PLOS is a purely xe2x80x9cfire and forgetxe2x80x9d system. Before firing, the operator estimates the angular velocity at which the line of sight is turning in the sight towards a moving target. The angular velocity is measured by the missile""s speed gyro and an estimator. Based on the estimated angular velocity, the target""s position is predicted as a function of time after firing and the missile is guided towards the predicted position of the target. At the same time, the effect of the earth""s attraction is eliminated. However, there are a number of error sources which limit the PLOS guidance method and which mean that the predicted position of the target does not always agree with the actual position of the target. Errors of the kind listed below can cause the missile to deviate from the desired point of impact or overfly point.
Error in sighting upon launch at target,
Error in estimating the angular velocity of the line of sight,
Error due to angular velocity having been assumed to the constant,
Error in missile""s control loop,
Error caused by incorrect estimation of disturbances from the environment, such as wind, etc.,
Error caused by inadequacies in missile and sensors.
Against the background of the above, there is therefore a need to increase the strike accuracy at greater distances which, in this context, can be distances in the range of 300 to 1000 meters, for example.
The object of the present invention is to improve the strike accuracy for PLOS-based guidance methods. The object of the invention is achieved by a method characterized in that the operator, in a second subsequent time period, tracks the actual position of the missile in relation to the predicted angle position of the target so that, if a deviation is observed, a correction command can be transmitted to the missile in order to correct the trajectory predicted for the missile, and also a guidance system which is characterized in that a communications link is provided to transmit any correction commands from the operator to the missile in a second subsequent time period for correction of the trajectory predicted for the missile.
In principle, according to the invention, the missile is driven autonomously after it has been fired. The missile does not need to be continuously fed from the sight with error positions. The firing is preceded by a phase where the angular velocity of the target is determined on the basis of the operator tracking the target in the time period between a first angle position and a second angle position. The tracking can be carried out optically, for example using visible light or IR light.
Since the operator has the possibility of continuously tracking the missile to the target and acting on the missile""s trajectory, the operator, if he considers that the missile is not lying within an acceptable distance from the line of sight, can introduce a correction which moves the missile towards the line of sight. The possibility for the operator to track and correct the missile""s course means that the errors in the above list can at least partially be compensated. The introduction of the correction during the missile""s travel towards the target increases the chances of firing at longer distances and striking rapid and/or manoeuvred targets.
It should be noted here that if the target is stationary, the operator can fire the missile directly at the target. Here again, he has the possibility of correcting deviations in missile trajectories.
According to an advantageous embodiment of the method, the missile trajectory is corrected during the second time period in steps in the direction counter to the observed deviation upon receipt of a correction command activated by the operator. An advantageous embodiment in this connection is characterized in that the correction of the missile trajectory during the second time period in the direction counter to the observed deviation is carried out in one or two steps. A correction in one or two steps is what a qualified operator is considered to be able to do under stress from enemy fire and the forces which are developed during the launch procedure.
According to another advantageous embodiment, an angular velocity of the target estimated in the first time period is corrected in the second time period, the missile trajectory being corrected in proportion to the firing distance, resulting in a stepwise correction in the direction counter to the observed deviation upon receipt of a correction command activated by the operator.
Correction of the missile trajectory is advantageously based on correction commands transmitted by the operator for target distances greater than 300 meters.
An advantageous embodiment of the guidance system according to the invention is characterized in that the communications link on the transmit side is connected to the firing mechanism of the missile via a decoder which, based on correction commands in the form of depressions of the firing mechanism by the operator, identifies the correction commands and, via a transmitter, sends the information to the missile. The guidance system does not require any extra input members on the transmit side of the communications link, and instead the correction commands can be fed via the same trigger which is used for determining angular velocity and for firing. This facilitates the operator""s handling of the weapon and means that soon after firing he can track the missile trajectory in order to effect possible correction.
Located on the receive side of the missile, the communications link, in an advantageous embodiment, comprises a receiver for receiving the correction commands and a computer unit connected to the receiver. The computer unit is preferably arranged to use ordinary algorithms to guide the missile in the desired predicted trajectory via a control device incorporated in the missile, preferably with hot gas propulsion via controlled valves or with aerodynamic control surfaces, based on received correction commands and information from the missile""s inertia sensors.
In an advantageous embodiment, the communications link of the guidance system operates with laser light.